Feature/optional build directx samples

Removes DirectX linking from all samples on Windows builds except the DirectX sample, so building samples without DirectX isn't an issue.

Fix auto-detects DirectX installation and builds sample if BUILD_SAMPLES=ON and as long as SKIP_DIRECTX_SAMPLES and SKIP_DIRECTX9_SAMPLE are not ON.

This patch is structured in such a way that more DirectX samples will be added, specifically the pending DirectX 10 example in libRocket PR #56.

Tested with Ubuntu/OSX to confirm only applies to Windows.
Tested with Windows 7, with and without DirectX SDK from June 2010 (last SDK before moving it into Windows Kit).
Tested with MinGW32 and Visual Studio 2012 with and without DXSDK installed.
Deals with partial SDK installations that are included in the full install of VS2012 Professional and higher SKUs